1. Field of the Invention
The present invention relates to an image reading apparatus which is capable of performing .gamma.-correction and background gray level correction in a facsimile machine or a photocopying machine for example. The present invention also relates to an information storing medium which is used for controlling such an image reading apparatus.
2. Description of the Related Art
Typically, an image reading apparatus, such as an image scanner of a facsimile machine, comprises an image reader for reading images of an image carrying paper for output of corresponding analog image signals, an A/D converter for converting the analog image signals into multi-bit digital image data, a .gamma.-correction unit for performing .gamma.-correction of the digital image data. Further, the image reading apparatus also includes means for correcting the background gray level of the image signals at the time of converting the analog image signals into the digital image data. Such background gray level correction is necessary because the dynamic range of the non-background image signals relative to the background image signals reduces to result in poor resolution if the background is relatively dark.
For background gray level correction, a conventional image reading apparatus incorporates a peak hold circuit for holding the peak level of analog image signals from an image reader, and the peak level held by the peak hold circuit is utilized as a high level reference value for an A/D converter. Thus, when the voltage of the analog image signals becomes equal to the peak value, the digital output of the A/D converter becomes equal to the high level reference value which is the highest value of the converter.
Another image reading apparatus is also known which incorporates an automatic gain control circuit including an amplifier for amplifying analog image signals to such a degree that the peak level of the analog image signals is elevated to coincide with the high level reference value of an A/D converter.
However, either of the image reading apparatuses described above requires hardware addition of the peak hold circuit or the automatic gain control circuit, consequently resulting in a cost increase.